This invention relates to an improved electrically alterable recording medium. More particularly, this invention relates to an electrically alterable holographic recording medium which does not fatigue after numerous write-erase cycles.
Phase holograms can be formed on a heat softened thermoplastic surface which selectively deforms during exposure to an applied charge pattern, as has been disclosed by Urbach in U.S. Pat. No. 3,560,205. According to this system, a corona discharge device connected to a suitable recording medium ionizes the air near the surface of the thermoplastic whereupon positive ions are deposited uniformly on the surface of the thermoplastic. This surface is now exposed to an image by means of coherent light split into an object beam and reference beam in known manner. The light interacts with a photoconductor which causes a redistribution of the charge in the areas where it impinges on the thermoplastic. When the thermoplastic is charged again, the electric field increases in the previously illuminated areas. The thermoplastic is then exposed to a temperature sufficient to soften its surface which deforms according to the electric field, becoming thinner or forming valleys in the areas of high field intensity. When cooled to room temperature, a hologram is recorded as a thickness variation or pattern in the thermoplastic. Such holograms can be erased in the absence of exposure and corona discharge by heating the thermoplastic above its softening point to a temperature sufficient to allow the surface tension of the thermoplastic to revert to its undeformed state.
This holography system would be highly useful in optical memory devices due to its ability to write and erase a series of holograms in situ. However, the thermoplastics employed heretofore gradually degrade or show fatigue after repeated write-erase cycles, when sharp, clear holograms can no longer be formed. In some cases, fatigue is discernible after several write-erase cycles.
Electrically alterable recording media described as suitable for recording deformation holograms comprise conductive substrates, a photoconductive insulating layer over the substrate and a deformable insulating thermoplastic layer over the photoconductor. The thermoplastics suggested heretofore for this system include natural resins, such as glycerol and pentaerythritol esters of partially hydrogenated rosin, and synthetic polymers such as p-alpha-methylstyrene, terpolymers of styrene, indene and isoprene, polyterpene resins from .beta.-pinene, olefin-diene resins, styrene-butadiene resins, polystyrene, coumaroneindene resins, chlorinated polyphenyl resins, styrene-acrylate copolymers, vinyltoluene-acrylate copolymers, alkyd resins, mixtures of styrene and silicone resins and unreactive phenol-formaldehyde resins.
These materials show fatigue after repeated write-erase storage cycles; in many cases the onset of fatigue is noted after a few hundred cycles. In order to be able to be incorporated into a practical optical memory system, the recording medium must be immune to fatigue over numerous write-erase cycles. Thus the search for an improved electrically alterable or deformable recording medium for holographic storage has continued.